Imaging paper, particularly photographic imaging paper, requires materials in the image substrate that provide long-term survivability and stability during both display and storage. These properties are most desirable and have significant commercial value.
It has been proposed in U.S. Pat. No. 5,244,861 to utilize biaxially oriented polypropylene sheets laminated to cellulose photographic paper for use as a reflective receiver for the thermal dye transfer imaging process. In the formation of biaxially oriented sheets described in U.S. Pat. No. 5,244,861, a coextruded layer of polypropylene is cast against a water cooled roller and quenched by either immersion in a water bath or by cooling the melt by circulating chill liquid internal to the chill roll. The sheet is then oriented in the machine direction and in the transverse direction. While a variety of materials may be used to create a biaxially oriented sheet, one of the preferred materials is polypropylene because of its strength and processing properties during the orientation. In addition, the low cost of this material makes it attractive to use.
In U.S. application Ser. No. 08/862,708 filed May 23, 1997, it has been proposed to use biaxially oriented polyolefin sheets laminated to photographic grade paper as a photographic support for silver halide imaging systems. In U.S. application Ser. No. 08/862,708 filed May 23, 1997, advantages including increased opacity, improved tear resistance, and reduced substrate curl are obtained by the use of high strength biaxially oriented polyolefin sheets. The above advantages of biaxially oriented polypropylene layers are realized when an opacifying pigment is located in at least one layer of polypropylene, which may be solid or voided. Either the rutile or anatase crystalline form of titanium dioxide (TiO.sub.2) is commonly used for opacity, whiteness, image sharpness, and control of pearlescence.
Polypropylene is inherently more susceptible to chemical degradation that leads to loss of mechanical properties. It undergoes thermal degradation during processing such as extrusion of thin films, and photooxidative degradation with long-term exposure to light. TiO.sub.2 catalyzes and accelerates both thermal and photooxidative degradation. In the art of resin coating photographic papers and also in the thermal processing of biaxially oriented polyolefins sheets, the melt polymers are extruded at high temperatures and are also subjected to high shear forces. These conditions may degrade the polypropylene resin, resulting in resin discoloration and charring, formation of polymer slugs, and formation of lines and streaks in the extruded film from degraded material deposits on die surfaces. Also, thermally degraded polypropylene is less robust than undegraded polymer for long-term stability, and may thereby shorten the life of the print.
Hindered phenol antioxidants are commonly used alone or in combination with secondary antioxidants to stabilize polypropylene during melt processing, but provide little protection from long-term photooxidation. They are also responsible for some forms of oxidative atmospheric gas yellowing in prints stored in the dark. This undesirable color may develop on the print or around the print edge with archival keeping, and has been attributed to colored oxidation products of hindered phenol antioxidants that are formed in the dark with exposure to oxidizing pollutants such as oxides of nitrogen in the presence of white pigments such as TiO.sub.2.
In U.S. Pat. No. 4,582,785 it is suggested that polymeric hindered amines, when added to polyethylene coated photographic paper, can improve their photostability. In this patent a polymeric hindered amine is claimed as the sole stabilizer for both thermal processing and light stability in a single layer of a polymeric material, polyethylene, that is inherently more stable than polypropylene to degradation. Photostabilizers such as the polymeric hindered amine improve the archival qualities of the resin layer by eliminating the phenolic antioxidant yellowing and preventing photo-degradation; however, while hindered amines provide adequate stabilzation of polyethyelene, they do not stabilize polypropylene significantly during extrusion, thereby severely limiting the latitude of processing conditions. The desired stabilizer package would contain both a hindered phenol for protection during extrusion, and a hindered amine light stabilizer for long-term photo stability. Unfortunately, the use of hindered phenols and hindered amine light stabilizers in a monolayer white imaging element is unacceptable because hindered amine light stabilizers worsen phenolic antioxidant discoloration.
There remains a need to provide an imaging support that contains a biaxially oriented, pigmented polyolefin sheet that is extrusion processable without degradation of polypropylene resin. In addition it must have exceptional long-term resistance to degradation and embrittlement when exposed to light and other environmental stresses, while providing an imaging support that has exceptional dark stability and prevents discoloration during dark keeping. The chemistry to achieve thermal processability and to maximize the life of images for light stability and dark keeping may require synergistic effects from more than one additive.